Nuclear power remains an important energy resource throughout the world. Many countries that lack adequate indigenous fossil fuel resources rely primarily on nuclear power to produce electricity. In many other countries, nuclear power is used as a competitive source of electricity which also increases the diversity of the types of energy used. In addition, nuclear power also makes a very important contribution to the achievement of such goals as controlling fossil fuel pollution (such as acid rain and global warming) and conserving fossil fuel for future generations.
Although safety is certainly a major issue in the design and operation of nuclear reactors, another key issue is the danger of the proliferation of materials that could be used in nuclear weapons. This danger is especially relevant to countries with unstable governments, whose possession of nuclear arms could pose a significant threat to world security. Nuclear power therefore should be generated and used in a way that does not lead to the proliferation of nuclear weapons and the resulting risk of their use.
All current nuclear reactors create large amounts of material customarily referred to as reactor-grade plutonium. A typical 1000 MW reactor, for example, creates about 200-300 kg per year of reactor-grade plutonium, which can be suitable for producing nuclear weapons. Hence the fuel discharged from the cores of conventional reactors is highly proliferative material, and security measures are required to prevent the discharged fuel from falling into the hands of unauthorized individuals. There is a similar security problem with the enormous stockpiles of weapons-grade plutonium created in the U.S. and the countries of the former Soviet Union in the process of dismantling of nuclear weapons.
There are other problems in the operation of conventional nuclear reactors associated with the constant need to dispose of long-life radioactive waste and the rapid depletion of worldwide supply of natural uranium raw material.
To solve these problems, there have been recent attempts to develop nuclear reactors that use relatively small amounts of nonproliferative enriched uranium (enriched uranium has a U-235 content of 20% or less) and do not generate significant amounts of proliferative materials such as plutonium. Examples of such reactors have been disclosed in international applications WO 85/01826 and WO 93/16477, which disclose seed-blanket reactors that obtain a substantial percentage of their power from blanket zones with thorium fuel. The blanket zones surround a seed zone containing fuel rods of nonproliferative enriched uranium. The uranium in the seed fuel rods releases neutrons which are captured by the thorium in the blanket zones, thus creating fissionable U-233, which burns in place and releases heat for the reactor power plant.
The use of thorium as nuclear reactor fuel is attractive because worldwide thorium reserves are considerably larger than uranium reserves. In addition, both of the aforementioned reactors are “nonproliferative” in the sense that neither the initial fuel loaded nor the fuel discharged at the end of each fuel cycle is suitable for producing nuclear weapons. This result is achieved by using only nonproliferative enriched uranium as seed fuel, selecting moderator/fuel volume ratios to minimize plutonium production, and adding a small amount of nonproliferative enriched uranium to the blanket zone, where the U-238 component is evenly mixed with the residual U-233 at the end of the blanket cycle and “denatures” (changes the natural properties of) the U-233, as a result of which it becomes unsuitable for making nuclear weapons.
Unfortunately, neither of the aforementioned reactor designs is truly “nonproliferative.” In particular, it has been discovered that both of the designs result in a level of production of proliferative plutonium in the seed zone which is higher than the minimum possible level. The use of a circular seed zone with both an inner or central blanket zone and an outer, surrounding blanket zone cannot provide reactor operation as a “nonproliferative” reactor, since the thin, annular seed zone has a correspondingly small “optical thickness,” which results in a seed (neutron) spectrum which dominates the considerably harder spectrum of the inner and blanket zones. This results in a higher proportion of epithermal neutrons in the seed zone and production of a higher than minimum quantity of proliferative plutonium.
In addition, neither of the previous reactor designs has been optimized from the standpoint of operational parameters. For example, moderator/fuel volume ratios in the seed zone and blanket zones are particularly critical for minimizing the amount of plutonium in the seed zone, so that adequate heat is released by the seed fuel rods, and optimum conversion of thorium to U-233 in the blanket zone is ensured. Research shows that the preferred moderator/fuel ratios indicated in the international applications are too high in the seed zones and too low in the blanket zones.
The previous reactor core designs also are not especially effective in consuming nonproliferative enriched uranium in the seed fuel elements. As a result, the fuel rods discharged at the end of each seed fuel cycle contained so much residual uranium that they had to be reprocessed for reuse in another reactor core.
The reactor disclosed in application WO 93/16477 also requires a complex mechanical reactor control system which makes it unsuitable for refitting a conventional reactor core. Similarly, the reactor core disclosed in application WO 85/01826 cannot easily be transferred into a conventional core, because its design parameters are not compatible with the conventional core parameters.
Finally, both of the previous reactor designs were designed specifically to burn nonproliferative enriched uranium with thorium and are not suitable for consuming large amounts of plutonium. Hence neither design provides a solution to the problem of stockpiled plutonium.
A reactor with a core which includes a set of seed-blanket assemblies, each of which contains a central seed region which includes seed fuel elements made of a material capable of nuclear fission containing uranium-235 and uranium-238, an annular blanket that surrounds the seed region and includes blanket fuel elements containing primarily thorium and 10% by volume or less enriched uranium, a moderator in the seed region, with a volume ratio of moderator to fuel in the range of 2.5 to 5.0, and a moderator in the blanket region, with a ratio of moderator to fuel in the range of 1.5 to 2.0, is known according to patent RU 2176826. Each of the seed fuel elements is made of uranium-zirconium alloy, and the seed zone makes up 25-40% of the total volume of each seed-blanket module.
The known reactor provides optimum operation from the standpoint of economy and is not “proliferative.” This reactor can be used to consume large amounts of plutonium with the thorium without generating proliferative wastes. The reactor produces substantially smaller amounts of hot waste, which significantly reduces the need for long-term waste storage sites.
However, the seed-blanket assemblies used in the reactor are not suitable for use in existing light water reactors such as the VVER-1000.
A fuel assembly for a light water reactor similar to the reactor described above, which, specifically, has a hexagonal cross-sectional form, which makes it possible to install the fuel assembly from the seed-blanket modules in a conventional light water reactor, is known from the description for patent RU 2222837.
Other than the presentation of the cross-sectional form of the assembly, however, the description for the aforementioned patent contains no information on the configuration of the assembly which would allow installing it in an existing light water reactor such as the VVER-1000 without modifying the reactor design.
A fuel assembly for a light water reactor including a bundle of fuel elements and guide channels in spacer grids, a tailpiece and a head, wherein the spacer grids are connected to each other and to the tailpiece by elements arranged along the length of the fuel assembly, and the head is made up of upper and lower tieplates, cladding situated between the plates, and a spring unit, and wherein outer ribs on the head shell are connected to each other along projections of the rim and along the lower parts by perforated plates, is known according to patent RU 2294570.
The known fuel assembly is classified as a design for jacketless fuel assemblies, which make up the cores of pressurized water reactors (PWRs) such as the VVER-1000, and has enhanced operating properties due to increased rigidity, reduced head length and increased free space between the fuel rod bundle and the head, with a simultaneous increase in the length of the fuel rods. This design makes it possible to increase the fuel load in the fuel assembly with greater depletion depth and thereby to increase the reactor core power and the life cycle of the fuel assembly.
However, all the fuel elements in this assembly are made of the fissionable material traditionally used in reactors such as the VVER-1000; consequently, the creation of large amounts of reactor-grade plutonium is a characteristic drawback of reactors with such assemblies.
One object of one or more embodiments of the invention is the creation of a fuel assembly which, on the one hand, generates a substantial percentage of its power in a thorium-fueled blanket region and does not create proliferative wastes and, on the other hand, can be installed in an existing light water reactor such as the VVER-1000 without requiring substantial modifications.